The Bug:

Underwater structures can spawn too close to the surface thus causing water blocks to generate above sea level. This can result in the said water in some cases not flowing.

Here is an example:

Seed: 2812221936213207833
Coordinates: /execute in minecraft:overworld run tp @s 19676.22 66.00 -606.09 105.14 30.45

Steps to Reproduce:

1. Generate a world with the seed provided above and teleport to the given coordinates.
2. Observe your surroundings and look at the underwater structure closely.
3. Take note as to whether or not underwater structures can generate water above sea level

Observed Behavior:

Underwater structures can generate water above sea level.

Expected Behavior:

Underwater structures would not be able to generate water above sea level.